1. Field of the Invention
The invention relates to a drying section of a machine for producing a material web, for example, a paper or cardboard web, and to methods of drying a material web in such a drying section, wherein the drying section comprises at least one drying group with at least one drying cylinder and comprises at least one web guidance device around which the material web is guided in a meandering manner. The invention further relates, in some aspects, to a roll that can be used particularly as a web guidance device in such a drying section.
2. Discussion of Background Information
Drying sections of the type specified above are known. They comprise a number of drying cylinders and web guidance devices, usually web guidance rolls, around which the material web is guided in a meandering manner. Preferably, the material web is firmly clamped between a transport or traveling screen and the drying cylinders in the area of the drying cylinders. In the area of the web guidance devices, the web is commonly fixed to the transport or traveling screen by means of a vacuum. This allows for runability in the drying section; however, shrinkage of the material web is hindered. Moreover, the steep shrinkage gradients at the edges, caused by shrinkage-hindered drying, lead to thicker edges with a rougher surface. This leads to problems during subsequent processing of the material web, for example in coating groups, printing machines, etc. Providing open sectioning points between the various drying groups is already known, as is reducing or switching off the vacuum, whereby the material web is less strongly fixed in position by lengths, and can shrink. The problems specified above, however, are not solved as desired in this manner.
In a drying section of the type specified above as disclosed in German patent document DE-U-296 16 569.7, several web guidance devices are respectively structured so that a gaseous medium can be supplied in several zones in sequence along the direction in which the web travels, in order to increase moisture removal from the web by blowing a gaseous medium on the material web, and thus to ensure more effective drying of the material web. However, in this drying section, the conveyor belt is removed from the material web before the material web winds onto the first web guidance device in the direction in which the web travels and before the material web is wound onto several subsequent web guidance devices, and is brought back together with the belt only after the material web reels off from the respective web guidance device. This entails problems particularly at the start of the drying section, where the strength of the wet web is still relatively low, which hinder the highest possible runability that is sought.